If your mom's only income is from Social Security there is no need or reason for her to file tax return. Unless she has taxes withheld from the SS benefits received, then she should file to get a refund of the taxes.
does she have other income? if not then the other advice is correct.
@SarahJ12 and the advice is the same even if Mom is not a dependent of Daughter....
1. If a person's only income is social security, it is not taxable and a return does not need to be filed unless tax was withheld and the person wants it refunded.
2. On the other hand, if a person has other taxable income in their own name (IRA, pension, gambling winnings, etc.) they must file a tax return in their own name even if they are also someone else's dependent.
3. Your sister can't claim your mother as a dependent if your mother has more than $4300 of taxable income (for 2021. I don't know the amount for 2022 but it will be a little higher.)
4. Even if your mother has less than $4300 of taxable income and can be claimed as a dependent by your sister, she is not a qualifying person for HOH unless she also lived in your sister's home more than half the year.